To be determined.
Cell migration, particularly migration of cancerous cells and nerve cells, is not well understood, nor are the factors that affect cell migration and tissue shaping in vivo. There is a need in the art to identify and exploit such factors, including but not limited to those involved in normal or abnormal organogenesis. The art also lacks efficient systems for evaluating therapeutic modulators of such functions in vivo and lacks diagnostic methods for assessing the ability of a cell or cell mass to migrate in vivo.
Organogenesis processes in vertebrates proceed in a manner similar to those observed in the common laboratory nematode C. elegans. As such, the generation of C. elegans gonadal structures can serve as a simple system for investigating developmental morphogenetic processes shared by higher and lower organisms.
In one common morphogenetic process, a tissue bud extends to form an elongate tube with a proximal to distal axis. An emerging theme in bud extension is the presence of specialized regulatory cells at the bud tip that govern elongation. In vertebrate development, this process is seen in extension of the limb (Johnson and Tabin, 1997; Martin, 1998), ureter (Vainio and Muller, 1997), and lung branches (Hogan, 1998). In the C. elegans gonad, long xe2x80x9carmsxe2x80x9d develop by elongation of buds originating from a gonadal primordium. Each gonadal arm possesses a single xe2x80x9cleader cellxe2x80x9d that serves this regulatory role (Kimble and White, 1981). The biology of distal tip cell migration during gonadogenesis is known to one skilled in the art of C. elegans developmental biology. Indeed, the C. elegans gonadal leader cells are among the best defined cells that regulate bud elongation, and therefore serve as a paradigm for investigating this common morphogenetic process.
A second common morphogenetic process of organogenesis is the formation of a complex, differentiated epithelial tube. Formation of a complex epithelial tube can involve an initial condensation of mesenchymal cells, followed by epithelialization, lumen formation, and differentiation into modular units. Vertebrate examples include the kidney tubules (Vainio and Muller, 1997) and heart tube (Fishman and Olson, 1997). Similarly, during C. elegans gonadogenesis, cells coalesce to form a compact larval structure called the somatic gonadal primordium (SGP). Following formation of this primordium, cell division and differentiation are accompanied by epithelialization and lumen formation to form a complex tube composed of distinct modular units: the uterus, spermathecae and sheaths in hermaphrodites, and the seminal vesicle and vas deferens in males (Kimble and Hirsh, 1979).
Previous studies have identified several genes in C. elegans that influence gonadal morphogenesis. One group of such genes includes unc-5, unc-6, and unc-40, which control the direction of leader cell migration (Hedgecock et al, 1990). Normally, leader cells migrate in one direction, then move dorsally, and finally move in the opposite direction to generate a reflexed gonadal arm. In the absence of unc-5, unc-6, or unc-40, the leader cells fail to turn dorsally. Another gene, ced-5, causes the leader cell to makes extra turns or stop prematurely (Wu and Horvitz, 1998). Therefore, in these mutants, the leader cells migrate, but do not navigate correctly, which results in a failure of the gonadal arms to acquire their normal U-shape. In addition to these genes, others are required for specification of cell fates and also influence morphogenesis (lin-12: Greenwald et al., 1983, Newman et al., 1995; lin-17: Sternberg and Horvitz, 1988; lag-2: Lambie and Kimble, 1991; ceh-18: Greenstein et al., 1994, Rose et al., 1997; lin-26: den Boer et al., 1998).
A known C. elegans genetic locus, gon-1, defined by one or more mutants, is essential for extension of gonadal germline arms, but is not responsible for signaling the germline to proliferate. In C. elegans hermaphrodites, GON-1 is required for migration of two distal tip cells to produce two elongated tubes, whereas in males, gon-1 activity is required for migration of a single linker cell to produce a single elongated tube. In gon-1 mutant hermaphrodites, the leader cells are born normally in the somatic gonadal cell lineage and function normally to promote germline proliferation, but they fail to migrate and do not support arm extension. Similarly in males, the leader cell does not move and no arm extension occurs. The gon-1 locus has not heretofore been mapped with particularity to a nucleic acid coding sequence.
Clarification of the genetic basis for C. elegans gon-1 activity would permit one to apply molecular tools to the study of cell migration in a convenient system. It would be particularly advantageous to find that the gon-1 locus encodes a protein having structural relationship to proteins of species that are not readily studied in the laboratory, since one would be able to evaluate those proteins in the convenient C. elegans system. Such a system would also provide a means for evaluating agents that can modulate the activity of such genes and proteins and would both facilitate understanding the factors involved in cell migration.
In one aspect, the invention can be an isolated polynucleotide coding sequence that encodes a protein the includes both a metalloprotease domain and at least one thrombospodin type 1 domain, where the protein can direct either cell migration or tissue shaping in an analytical system in a target organism as disclosed herein. In another aspect, the invention can also be a variant of the isolated polynucleotide coding sequence that encodes a protein that shares at least 20%, more preferably 50%, still more preferably 70% and most preferably 80% amino acid sequence identity (using GCG Pileup program) with any of the foregoing in the metalloprotease and thrombospondin type 1 domains while also comprising the amino acids of those domains known to those skilled in the art to be required for protein activity. A suitable variant polynucleotide can hybridize under stringent hybridization conditions known to those skilled in the art to a polynucleotide sequence that encodes a protein that can direct cell migration or tissue shaping in the target organism. In one embodiment, a variant polynucleotide can hybridize under stringent hybridization conditions to a C. elegans gon-1 coding sequence. The variant polynucleotide sequence can be a polynucleotide obtained from an organism or can be a mutated version of any polynucleotide sequence noted above. The variant polynucleotide can encode a protein that is identical or altered relative to the wild-type C. elegans GON-1 protein. The encoded protein can have enhanced or reduced activity in vivo relative to GON-1.
In a related aspect, a polynucleotide coding sequence that encodes a protein having structural and functional similarity with a wild-type or altered migration or shaping protein can also be substituted, in whole or in part, with structurally related or unrelated sequences to encode a heterologous protein or a chimeric protein in the disclosed system, as detailed below.
Applicants herein disclose that the Caenorhabditis elegans gon-1 activity is encoded by a polynucleotide coding sequence (gon-1; SEQ ID NO:1) that encodes an essential protein (GON-1; SEQ ID NO:2) that directs migration of a growing gonadal tube through surrounding basement membranes during gonadogenesis in the nematode and also controls gonadal shape and organ localization.
The migration directing ability and tissue shaping ability are separable and depend upon whether the gon-1 coding sequence is expressed in distal tip cells or in muscle cells, respectively. In wild-type C. elegans, a gonad of normal shape is produced when gon-1 is expressed in both cell types. Accordingly, one aspect of the invention can also a method for shaping a tissue by selectively expressing a protein associated with both tissue elongation and tissue expansion. GON-1 shares significant amino acid identity with proteins that have been noted in other species.
In a related aspect, the invention can be an isolated and substantially purified preparation of a GON-1 protein, an altered GON-1 protein, a heterologous protein, a chimeric protein, or a variant thereof (referred to herein as xe2x80x9can MPT proteinxe2x80x9d, for reasons discussed below), which can be a target for in vivo screening of putative therapeutic modulators, or can be assayed in a diagnostic method for assessing the ability of a cell or cell mass to migrate in vivo, or can be exploited as a therapeutic agent to modulate (increase or decrease) in vivo cell migration.
One skilled in the art will appreciate that the nucleotide coding sequences and encoded amino acid sequences that fall within the scope of the invention are also subject to natural variation or intentional manipulation (e.g., changes in the nucleotide or amino acid sequence) in ways that do not affect the ability to function as described herein. One skilled in that art also understands that the applicants cannot provide a complete list of nucleotide coding sequences and amino acid sequences that can function in the methods of the invention. However, in view of the high level of understanding in the art about the amino acids required for activity of proteins that comprise a metalloprotease domain and proteins that comprise a thrombospondin domain, applicants maintain that a skilled artisan can readily determine whether a protein contains both domains. Stxc3x6cker, W. et al., xe2x80x9cThe metzincinsxe2x80x94Topological and sequential relations between the atacins, adamalysins, serralysins, and matrixings (collagenases) define a superfamily of zinc-peptidases,xe2x80x9d Protein Science 4:823-840 (1995), Rawlings, N. D. and A. J. Barrett, xe2x80x9cEvolutionary families of metallopeptidases, Methods in Enzymology 248:183-228 (1995), and Adams, J. C. et al., The Thrombospondin Gene Family, R. G. Landes Company, Austin, Tex. (1995), all incorporated herein by reference in their entirety, provide sufficient guidance to permit those in the art to establish whether a protein comprises both a metalloprotease and a thrombospondin domain.
The invention is further summarized in that an antibody can be produced against characteristic epitopes of any of the foregoing proteins using standard methods. The antibody can be used both diagnostically to ascertain the presence of an MPT protein, or therapeutically to interfere with activity of the MPT protein.
The present invention is also summarized in that an animal that contains a gon-1 allele (or homolog or variant thereof) is a convenient screening tool for finding modulators of cell migration. The present invention is thus further summarized in that a method for identifying modulators of the disclosed MPT proteins includes the steps of treating a target organism having a cell that can migrate or be shaped when under control of an MPT protein with at least one potential modulator of migration or shaping and observing in the treated target organism a change in migration or shaping of the cell or tissue attributable to the presence of a modulator. In a preferred embodiment, the cell is a developing gonadal cell in C. elegans, although other cells or organs may be similarly regulated by MPT proteins in other organisms.
The ability of the MPT protein to direct a cell or tissue under its influence to migrate or be shaped can be modulated (increased or decreased) in a variety of ways, such as by altering the migration protein""s primary, secondary, or tertiary structure, by altering the location or amount of the protein in an organism, by altering the transcriptional or translational regulation of the gene that encodes the protein, or by providing the organism with an agonist or antagonist molecule in an amount sufficient to interact with the MPT protein so as to increase or decrease the ability of the protein to direct migration or shaping.
In a related method, one can also identify nucleic acid sequences required or desired for migration or shaping of such a cell, by treating a target organism with an agent that affects the polynucleotide sequences of the target organism that encode the MPT protein or that participate in regulating expression of the MPT protein, and then identifying sequences affected by the treatment. The sequences identified in the method can be either complete or partial coding sequences or can be regulatory sequences.
It is an object of the present invention to identify a protein and nucleotide sequence encoding same that directs migration or shaping of a cell or tissue.
It is another object of the present invention to provide a method for modulating cell migration or shaping.
It is yet another object of the present invention to provide a system and method for screening putative modulators of migration or shaping of cells or tissues.
It is an advantage of the present invention that agents having a putative effect upon migration or shaping can be screened in a convenient model system rather than in a vertebrate organism.
Other objects, features and advantages of present invention will become apparent upon consideration of the following detailed description taken in conjunction with the accompanying drawings.